1. The problem statement, all variables and given/known data Prove that the set of all n x n matrices A such that AB = BA for a fixed n x n matrix B, is a subspace of Mnn. 2. Relevant equations u + v is in the same vector space as u and v. ku is in the same vector space as u, where k is any real number. 3. The attempt at a solution I am drawn to think of a diagonal matrix when I think of this question. And if I multiply a diagonal matrix by a scalar, it can only be a diagonal matrix or the zero matrix, either way, it AB still equals BA. Similarly, adding two diagonal matrices obtains either another diagonal matrix or the zero matrix...so, in this way, A is a subspace of Mnn. Am I correct?